Work is underway to tackle a disease that was an pandemic in the 14th century and has killed up to 200 million people - the Black Death.

While many may think that bubonic plague is something that only now exists in history books it is still rife in Madagascar, the Democratic Republic of Congo and Peru.

It is also still killing seven Americans a year - but scientists are looking for a human vaccine, a way of controlling it in animals and developing more effective ways of diagnosing it.

Disease: Public health workers treat the ground in Yosemite National Park to get rid of fleas (Image: Reuters)

The renewed interest may be because the plague is classified as a "category A bioweapon" and there are fears it could be used in biological warfare.

The disease, spread by fleas on animals, wiped out up to half the population of Europe from the 14th century with the most famous outbreak being the Great Plague of London in 1665.

There have been 15 Americans die of the disease in 2015 so far up from an average of seven in 12 months - which has steadily claimed over the past five years - according to the Center for Disease Control and Prevention (CDC) - and the figure of four deaths is the highest this century.

The bacterium behind the plague - yersinia pestis - arrived in the US in 1900 on rat-infested ships, said to Daniel Epstein of the World Health Organization (WHO).

He said: "Plague was pretty prevalent, with epidemics in Western port cities. But the last urban plague was in Los Angeles in 1925.

"It spread to rural rats and mice, and that's how it became entrenched in parts of the US."

It is still spread by animals such as prairie dogs and rodents.

If diagnosed early it can now be treated with antibiotics and animal vaccines have been created, the BBC reports.

Close look: Bubonic plague under the microscope (Image: Getty)

Fleas: The plague can be carried by prairie dogs (Image: Getty)

The "plague line" is New Mexico, Arizona, California and Colorado, according to Dr Amesh Adalja, an infectious-disease specialist at the University of Pittsburgh's Center for Health Security who say research into the problem is "vibrant".